Numerous electrochemical reactions are conducted in titanium vessels. Among such industrially significant processes are the electrolysis of dilute brines, e.g., as in the desalinization of water and the decomposition of water, and the electrolysis of concentrated brines such as in the production of alkali metal halates, e.g., sodium chlorate, and the production of halogens, such as chlorine.
In the production of chlorine by the electrolysis of water in a diaphragm cell, a brine containing from about 300 to about 325 grams per liter of sodium chloride is introduced into the anolyte chamber of the diaphragm cell. Chlorine gas, Cl.sub.2, is liberated at the anode within the anolyte chamber. The electrolyte, i.e., sodium chloride, then passes through the diaphragm to the catholyte chamber. Hydrogen gas, H.sub.2, is liberated at the cathode and sodium hydroxide, NaOH is collected in the catholyte chamber.
The anolyte typically has a pH of from about 3.5 to about 5, and typically contains from about 250 grams per liter to about 300 grams per liter of sodium chloride. The catholyte typically has a pH of from about 12 to about 14 and contains from about 120 to about 180 grams per liter of sodium chloride, and from about 110 to about 160 grams per liter of sodium hydroxide.
In electrolytic cells, the cell body is fabricated of electrolyte-resistant materials. For example, the catholyte body is fabricated of iron, steel, nickel, chromium, or other suitable catholyteresistant metals. The anolyte chamber body may be fabricated of a valve metal. The valve metals are those metals which form a protective oxide coating on exposure to acidic media under anodic conditions, such as titanium, tantalum, vanadium, niobium, and the like. Usually, in diaphragm cells for the electrolysis of brines, the catholyte chamber is fabricated of iron or steel, and the anolyte chamber may be fabricated of titanium.
The anolyte chamber, fabricated of titanium, contains various crevices, such as at joints, edges, seals, and the like. Titanium, in these crevices, is particularly susceptible to a form of corrosion characterized as crevice corrosion. While the exact mechanism of this form of corrosion is not fully understood, it is generally found only in thin crevices, characterized by a high ratio of metal surface area to electrolyte volume within the crevice. It is generally believed that crevice corrosion is caused by the diffusion or seepage of electrolyte through gasketing into the crevice, establishing a local cell within the crevice. It has been found by previous workers that the electrolyte within the crevice is highly acidic, generally having a pH of less than 2, for example of 1.5 or even as low as 1.0. Within such crevices, the concentration of corrosion products is high. Concentrations on the order of more than 10 grams per liter and even higher, e.g., as high as 20 or even 30 grams per liter, have been reported. Additionally, any iron present in the titanium appears to serve as a site for the crevice corrosion of titanium.
The cathodic side of the local cell within the crevice generally contains a titanium hydride or subhydride e.g., TiH.sub.2, phase which is brittle and readily flakes away to be hydrolyzed within the local cell. The anodic side generally contains incompletely formed suboxides of titanium which also flake away to form corrosion products which may be subsequently hydrolyzed.
The electrolyte within the local cell is further characterized in that it is oxygen deficient, and contains large amounts of halogen ion.
There have been various attempts to solve the problem of crevice corrosion; for example, various alloys of titanium with nickel such as the 2 percent nickel-titanium alloy disclosed in U.S. Pat. No. 3,469,975 to Bertea et al. Additionally, attempts have been made to reduce the surface iron content of the titanium such as disclosed in commonly assigned copending application Ser. No. 239,991 now U.S. Pat. No. 3,836,410 of Donald W. DuBois for "Method of Treating Titanium-Containing Structures." Other attempts at controlling crevice corrosion have included various coatings and have included the application of various surface films and coatings on the titanium and various treatments of the titanium surface.